1. Field of the Invention
The present invention relates to a semiconductor storage device.
2. Description of the Related Art
Resistive memory has attracted increased attention as a likely candidate for replacing flash memory. As described herein, it is assumed that the resistive memory devices include ReRAM (Resistive RAM), in a narrow sense, that stores its resistance states in a non-volatile manner using transition metal oxide as a recording layer, as well as Phase Change RAM (PCRAM) that uses chalcogenide, etc., as a recording layer with resistance information on its crystalline states (conductors) and amorphous states (insulators).
It is known that the variable resistance elements in resistive memory have two modes of operation. One is to set a high resistance state and a low resistance state by switching the polarity of the applied voltage, which is referred to as “bipolar type”. The other enables the setting of a high resistance state and a low resistance state by controlling the voltage values and the voltage application time, without switching the polarity of the applied voltage, which is referred to as “unipolar type”.
To achieve high-density memory cell arrays, the unipolar type is preferable. This is because that the unipolar type solution enables, without transistors, cell arrays to be configured by superposing variable resistance elements and rectifier elements, such as diodes, on respective intersections between bit lines and word lines. Moreover, large capacity storage may be achieved without an increase in cell array area by arranging such memory cell arrays laminated in a three-dimensional manner.
In such resistive memory, multi-value storage type memory has been proposed in an attempt to increase storage capacity where data of not less than 2 bits is stored in one memory cell by controlling resistance values of the variable resistance elements in one memory cell in a number of stages (see, for example, Japanese Patent Laid-Open No. 2006-4480).
However, such resistive memory of multi-value storage type has a problem that it is difficult for a sense amplifier circuit to control and provide a reference voltage with high accuracy. Any errors in the reference voltage can increase the likelihood that an erroneous read operation will occur in a memory cell in which multi-values have been stored.